"I calculate that small comets, capable of destroying a city, only hit the Earth once every 1000 years or so."

But the risk to Earth from larger comets, around 1 kilometre wide, has not changed.

Luckily such continent-busting comets, of the size seen in the movie Deep Impact, are much rarer.

The risk of these large comets posing a risk to Earth is up to one in every 150 million years, depending on where the comets originate.

Listen to the sound of a comet as it swings in from beyond Pluto and sunlight warms it up, causing molecules to vapourise from its surface (Sound: Paul Francis/Australian National University/mp3 file, 663.3kb)

Ridiculous estimates of comets

Francis says previous estimates on comet hazards were based on incorrect extrapolations from the comet reports of amateur astronomers.

In 1967 a US astronomer, Edgar Everhart, calculated that the two comets a year being picked by amateur astronomers represented only 3% of the comets actually out there.

Francis' suspicions about the accuracy of this estimate were roused when he used Everhart's calculations to estimate the number of comets that would be picked up by a new telescope being built at Mount Stromlo.